1. Field of the Invention
This invention relates to an earth current detector for detecting transient earth currents that occur in advance of an earthquake or the like.
2. Description of Prior Art
Over the years there have been many reports of anomalous phenomena being observed prior to the occurrence of earthquakes: abrupt changes in earth currents (earth potential) and the electrical resistance of earth crust, luminous phenomenon, abnormal animal behavior, and earthquake clouds, to name a few.
In Greece, earthquake prediction based on the detection of earth currents is being conducted with a fairly high rate of success. According to the Greek method, a number of electrode-pairs are buried several meters under ground at locations separated East-West and North-South by several tens or several hundreds of kilometers, and the electrical potential between pairs of these electrodes is monitored. The signals obtained are passed through a 0.1 Hz low-pass filter, converted from analog to digital and transmitted in real time via telephone circuits to a central observation point, where they are recorded and displayed. The occurrence of an earthquake is predicted when a change in potential exceeds a prescribed level. (P. Varotsos and M. Lazaridou; Tectonophysics, 188 (1991) P321-347.)
In Japan, an attempt is being made to use a measurement electrode consisting of a buried 600 m casing pipe and a total of 140 m of electrically conductive wire arrayed around the pipe as a measurement electrode for detecting, as earthquake precursors, anomalies in three radio wave frequency bands: DC--0.7 Hz, 0.01-0.1 Hz and 1-9 kHz. (Yukio Fujinawa, Kozo Takahashi, Sadaharu Kumagaya, Earthquakes Vol. 43, No. 2 (1990) P287-290.)
U.S. Pat. No. 4,904,943 teaches an earthquake prediction method in which earth currents are simultaneously observed at four points, the detected signals are processed to determine the origin and intensity distribution of the earth currents, and the hypocenter region, scale, and time of occurrence of an earthquake are predicted from changes over time in the origin and intensity.
U.S. Pat. No. 4,837,582 teaches a method of detecting radio waves generated by a hypocenter region. When the method is used on land, a linear antenna is buried to a depth of at least 1,000 m and a radial antenna centered on the linear antenna is laid out on the earth's surface. When applied in the sea, an insulated electrically conductive cable is laid on the sea bottom at a depth of 200 m or more for use as an antenna. In either case, the occurrence of earthquakes is predicted on the basis of radio waves picked up by the antenna.
The Greek method of earthquake prediction mentioned earlier is also being tested in Japan, but with less success in an industrialized and populated region. This is thought to be because of differences in the intensity of man-made electrical noise near cities. Moreover, since the measurements in this method are affected by both natural phenomena such as rain and lightning and by artificial electrical noise, it is necessary to rely on human judgment in determining whether or not a detected signal is an indication of an impending earthquake.
The graph of FIG. 6 shows the results obtained when an electrode was buried to a depth of 2 m at each of two locations, a rock dynamiting site and a point about 70 m away from the site, and the earth current between the two points was measured at the time rock was dynamited. As shown in FIG. 6(b), the explosion occurred about 23 seconds after the start of the measurement. As can be seen from FIG. 6(a), the earth current produced by the explosion could not be detected because it was masked by an interfering noise signal produced by commercial power line current frequency electromagnetic waves.
As will be understood from the foregoing, methods involving the measurement of the potential difference, electric current, or the resistance between two surface points have the drawback of being affected by unpredictable factors such as changes in the weather (e.g. rain and lightning) and man-made electromagnetic noise. Conditions thus vary from one measurement region to another. This means that different observers may, depending on their experience, come to different conclusions regarding whether a particular signal is caused by one of these disturbances or is a sign of an impending earthquake. Moreover, since the measurement instruments have their results of measurement recorded by a pen recorder, their sensitivity to fracture-induced current and other such rapidly fluctuating signals is low. In view of this situation, there is a need for an earth current detection method that is not affected by the climate and human activity in the region being monitored and is not dependent on the judgment of the observer.
In addition, when the aforesaid method of predicting earthquakes on the basis of low-frequency band electromagnetic waves is used in urban areas where the level of artificial electromagnetic noise is high, the effect of the noise has to be reduced by conducting the measurement at great depth underground because the electromagnetic disturbance from the man-made noise penetrates the ground to a considerable depth. Moreover, the need to use a large antenna for picking up the low-frequency electromagnetic waves results in high equipment costs.
Where earthquake detection is conducted on the basis of underground radio waves received by an antenna extending to a depth of 1,000 m or more below the ground surface as taught by the earlier mentioned U.S. patent, the cost of installing the antenna becomes prohibitively high at sites other than at abandoned mines and wells. The points at which measurement can be conducted are thus severely limited.